Murine mammary tumor viruses (MuMTV's) exist as both endogenous, vertically-transmitted, and exogenous, milk-transmitted viruses. All laboratory strains of mice have MuMTV proviral sequences integrated into their genomes. Expression of these endogenous MuMTV genes is the subject of investigation in our laboratory. In the BALB/c strain, there appears to be a transcriptional control that restricts the amounts of MuMTV RNA synthesized. In contrast, in the C57BL strain, MuMTV RNA is synthesized by the lactating mammary glands (LMG's), but there appears to be a translational control that leads to undetectable levels of MuMTV antigens in these tissues. We have defined the genetic loci responsible for MuMTV RNA synthesis in C57BL LMG's as the three unlinked, independently segregating proviruses endogenous to this strain. Control of translation of MuMTV mRNA in C57BL LMG's will be investigated by using cell-free translation system derived from rabbit reticulocytes. For purification of viral mRNA's, recombinant plasmids carrying fragments of MuMTV DNA will be generated and amplified in prokaryotic cells. These recombinant DNA molecules will be used for hybridization-selection methods to purify MuMTV mRNA's. In addition to these experiments, genetic control over endogenous MuMTV gene expression in C3HfC57 mice will be studied by measuring viral gene products in LMG's of hybrids and backcrosses between BALB/c or C57BL females and C3Hf males. These data will be correlated with the development of mammary tumors in these mice.